Formate dehydrogenase from cell extracts of M. vanniellii has been resolved into two forms by ammonium sulphate fractionation. Both forms, P-I and P-II, are oxygen sensitive, are stable at 60 degrees C, have optimal catalytic activity at pH 9.1, and are inhibited by cyanide. P-I has been purified to near homogeneity and has been shown to contain approximately 15 moles of iron and 2 moles of molybdenum per mole of enzyme. P-II contains selenium while P-I does not; P-II is absent in extracts of cells grown without selenite. SDS gel filtration chromatography shows P-I and P-II formate dehydrogenase in identical positions perhaps suggesting that P-II exists as a complex of P-I and selenoprotein. A low molecular weight yellow compound, F420, has been isolated from extracts of M. vannielii. F420 shows a blue-green fluorescence in ultraviolet light and an absorption peak at 420 nm when oxidized; the color, fluorescence and 420 nm peak disappear upon reduction. Partially purified extracts show formate dependent NADP reduction with an absolute requirement for F420. P-I but not P-II shows this identical formate-NADP oxidoreductase activity. BIBLIOGRAPHIC REFERENCES: J.B. Jones and T.C. Stadtman. Methanococcus Vannielii: Culture and effects of Selenium and Tungsten on Growth. J. Bacteriol. 130, 1404-1406 (1977). J.B. Jones, Blair Bowers and T.C. Stadtman. Methanococcus vannielii: Ultrastructure and Sensitivity to Detergents and Antibiotics. J. Bacteriol. 130, 1357-1363 (1977).